1. Field of the Invention
The present invention relates to the receiving technologies, and it particularly relates to method and apparatus for determining the gain from the received signals and amplifying the received signals by the determined gain.
2. Description of the Related Art
An OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme is one of multicarrier communication schemes that can realize the high-speed data transmission and are robust in the multipath environment. This OFDM modulation scheme has been used in the wireless LAN (Local Area Network) standards such as IEEE802.11a/g and HIPERLAN/2. The burst signals in such a wireless LAN are generally subject to the effect of frequency selective fading. Hence, a receiving apparatus generally carries out the channel estimation dynamically.
In order for the receiving apparatus to carry out the channel estimation, two kinds of known signals are provided within a burst signal. One is the known signal, provided for all carriers in the beginning of the burst signal, which is the so-called preamble or training signal. The other one is the known signal, provided for part of carriers in the data area of the burst signal, which is the so-called pilot signal (See Reference (1) in the following Related Art List, for instance).
Related Art List
    (1) Sinem Coleri, Mustafa Ergen, Anuj Puri and Ahmad Bahai, “Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems”, IEEE Transactions on broadcasting, vol. 48, No. 3, pp. 223-229, September 2002.
In wireless communications, adaptive array antenna technology is one of the technologies to realize the effective utilization of frequency resources. In adaptive array antenna technology, the amplitude and phase of signals transmitted from and received by a plurality of antennas, respectively, are so controlled as to form a directional pattern of the antenna. Hereinafter, such a directivity pattern will be referred to as “adaptive pattern”. One of techniques to realize higher data transmission rates by using such an adaptive array antenna technology is the MIMO (Multiple-Input Multiple-Output) system. In this MIMO system, a transmitting apparatus and a receiving apparatus are each equipped with a plurality of antennas, and a channel corresponding to each of the plurality of antennas is set. That is, channels up to the maximum number of antennas are set for the communications between the transmitting apparatus and the receiving apparatus so as to improve the data transmission rates. Moreover, combining this MIMO system with a technique such as the OFDM modulation scheme results in a higher data transmission rate.
The combination of antenna directivity patterns in the transmitting apparatus and the receiving apparatus in the MIMO system may be expressed as follows, for example. One case is that the antenna of the transmitting apparatus has an omni pattern whereas the antenna of the receiving apparatus has an adaptive pattern. Another case is that both the antenna of the transmitting apparatus and the antenna of the receiving apparatus have adaptive patterns. The system can be more simplified if the former is used whereas the antenna directivity pattern is controlled in a more detailed manner in the latter and therefore the characteristics thereof can be improved. In either case, when the receiving apparatus receives data, the gain needs to be controlled by AGC (Automatic Gain Control) before the aforementioned channel estimation is executed. For the control of gain, a known signal for use with AGC is generally assigned prior to a known signal for use in channel estimation, in a burst signal.
In the foregoing circumstances, the inventor came to recognize the following problems to be solved. In order to realize the coexistence of a system which is not compatible with MIMO (hereinafter referred to as “Legacy system”) and a MIMO system, each burst signal is such that a signal compatible with a MIMO system is assigned posterior to a signal compatible with a Legacy system. The signal compatible with a Legacy system is a signal which can be received even by a receiving apparatus in a Legacy system whereas the signal compatible with a MIMO system is a signal to be received by a receiving apparatus compatible with a MIMO system. In the above arrangement, the known signals for use with AGC are assigned in the beginning stages of the respective signals. That is, a known signal, for use with AGC, which is compatible with a MIMO system is placed posterior to a known signal, for use with AGC, which is provided for a Legacy system.
In the receiving apparatus, the gain determined by the known signal, for use with AGC, which is provided for a Legacy system differs generally from the gain determined by the known signal, for use with AGC, which is compatible with a MIMO system. In particular, when beamforming is performed on the signal compatible with a MIMO system, the difference therebetween becomes larger. In order for the receiving apparatus to receive the signals compatible a MIMO system under such a circumstance, a gain in response to the beamformed signals is required. However, if the period of a known signal in the beamformed signals is short, the error contained in the gain determined by the receiving apparatus becomes larger.